The present disclosure describes a scale-bar artifact used either to assess the performance of or determine the compensation parameters for a coordinate measurement device.
Generally speaking, any device that measures from one to six dimensions (degrees-of-freedom) of an object is generically referred to as a coordinate measurement device (CMD). Some coordinate measurement devices (CMDs) measure a single dimension only (for example, a distance or an angle); other devices measure three dimensions (for example, the rectangular coordinates of a point in space); and still other devices measure six dimensions (for example, the rectangular coordinates plus the pitch, yaw, and roll angles of a rigid body).
For most CMDs, two types of tests are performed at various times throughout the life of the device: the compensation tests and the performance-verification tests. The compensation tests are performed to determine numerical values known as compensation parameters. These numerical values are used by a computer, microprocessor, or similar computing device to remove small systematic errors and improve measurement accuracy of the CMD. The performance-verification tests are performed to ensure that the CMD meets its published performance specifications. Ideally, the performance-verification tests are carried out in such a way as to be traceable to the standards at a National Measurement Institute.
The laser tracker is a type of coordinate measurement device that can be used to measure three-dimensional coordinates in space. The laser tracker sends a laser beam to a retroreflector target that is held against a surface of interest or placed into a fixed nest. The most common type of retroreflector target is the spherically mounted retroreflector (SMR). The SMR comprises a cube-corner retroreflector mounted within a sphere with the vertex of the cube-corner at the sphere center. A gimbal mechanism within the laser tracker directs a laser beam from the tracker to the SMR. Part of the light retroreflected by the SMR enters the laser tracker and passes onto a position detector. A control system within the laser tracker uses the position of the light on the position detector to adjust the rotation angle of the mechanical azimuth axis and the mechanical zenith axis of the laser tracker to keep the laser beam centered on the SMR. In this way, the laser beam is able to track an SMR that is moved over the surface of an object of interest. Part of the light retroreflected into the laser tracker passes into a distance-measuring device (distance meter) such as an interferometer or absolute distance meter (ADM). Angular encoders attached to the mechanical azimuth and zenith axes of the tracker measure the azimuth and zenith angles of the laser beam (with respect to the tracker frame of reference). The one distance and two angles measured by the laser tracker are sufficient to completely specify the three-dimensional location of the SMR.
One system-level performance-verification test for a laser tracker involves locating a calibrated scale-bar (artifact) in a succession of different orientations. The most common orientations are horizontal, vertical, left-diagonal, and right-diagonal. At each orientation, the laser tracker measures the coordinates of two or more positions, defined on the bar by magnetic nests designed to hold an SMR. The laser tracker is moved away from the scale-bar to a variety of positions and is rotated into a variety of orientations. At each position and orientation, the length of the scale-bar is calculated from the coordinates measured by the laser tracker and compared to the reference length of the scale-bar, which is known to high accuracy. Usually, the maximum discrepancy between the measured and reference length that is allowable for any particular measurement depends on the specifications for the particular laser tracker and also on the geometry of the laser tracker relative to the scale-bar.
For the laser tracker, a performance-verification test is often performed for a distance-measuring subsystem (interferometer or ADM), as well as for the overall system. One way to verify the performance of a distance-measuring device within the tracker is to compare its readings to those of a reference interferometer. This comparison may be made by placing a retroreflector target that intercepts the laser beam from the tracker back-to-back against a retroreflector that intercepts a laser beam sent out by the reference interferometer. Usually, the target/retroreflector assembly is moved along a rail. At each point along the rail, the sum of distances measured by the interferometer and the distance-measuring device should be constant. Any discrepancy from a constant value is regarded as an error in the distance-measuring device of the tracker. At many facilities, it is impractical because of expense and time to set up an automated interferometer rail of the sort described above. In these situations, an alternative performance-verification procedure is needed.
Although most interferometers do not require compensation, many absolute distance meters (ADMs) have compensation parameters that must be determined, perhaps periodically, to maintain maximum accuracy. The nature of the compensation parameters depends on the technology of the particular ADM, but to be specific we consider the case of an ADM that determines distance by intensity modulating laser light with a sinusoidal waveform and then comparing the measured phase of the light bounced off the target to the measured phase of light traveling in a reference path within the ADM. In such a system, an error may be caused when laser light reflects off optical components and into the ADM or when there is electrical cross talk among electrical components in the system. Such errors are referred to as cyclic errors because they vary sinusoidally with distance from the laser tracker. The period of such deviations is usually equal to the ADM ambiguity range divided by an integer m=1,2, . . . . The ADM ambiguity range is equal to c/2fmng, where c is the speed of light in vacuum, fm is the frequency of modulation of the laser, and ng is the group index of refraction of the air through which the laser beam travels. In the case of a laser tracker that contains an interferometer as well as an ADM, it is easy to compare interferometer and ADM measurements to determine the coefficients that define the magnitude and phase of the cyclic errors. However, in systems that contain an ADM but not an interferometer, an alternative method is needed to determine the ADM compensation parameters.
Although a calibrated scale-bar is often required for the testing of coordinate measurement devices, including the three degree-of-freedom laser tracker and the one degree-of-freedom ADM, there are practical problems in obtaining calibrated scale-bars that are sufficiently accurate over the wide range of temperatures that are present in many factory environments and that are affordable and easy to use. Most scale-bars available today are characterized at only one temperature, which is usually near 20 degrees Celsius. The most accurate of the scale-bars are usually constructed out of materials with a low thermal coefficient of expansion (TCE) such as Invar, SuperInvar, or composite material. However, the TCEs for such materials vary widely, and even the best of these materials can be counted on to have a constant length only at temperatures near 20 degrees Celsius. In addition, all of the aforementioned materials are expensive. Invar and Superinvar are also heavy, and composites tend to absorb moisture from the air. Most scale-bars available today have been designed for a laboratory environment having a well controlled temperature and humidity. Devices such as laser trackers, however, are used on a factory floor with temperatures and other environmental conditions that differ substantially from those found in a laboratory. To evaluate the performance of such devices on the factory floor, it is advisable to perform the performance verification procedures on the factory floor. For similar reasons, it is also advisable to compensate ADMs on the factory floor.